The hippocampus plays an important role in the formation of memories. Damage to this brain region is related to memory impairment found in aging., Alzheimer's Disease, and amnesia. It has been shown that estrogen has an effect on the anatomy and activity in this brain region. The main objective of the proposed project is to investigate the dynamics of the firing patterns of normally cycling as well as in estrogen depletion animals. Specifically, we will examine (1) the effects of the absence of estrogen (ovariectomized females) and estrogen replacement on the hippocampal representation in a familiar experiment; (2) changes in firing patterns of hippocampal neurons that may occur across the rat's natural estrous cycle, while performing a simple motor tasks; (3) the influence of environmental cues on the firing patterns of hippocampal place cells on a more complex tasks that assesses working memory ability in naturally cycling animals. The findings from the present study will have important implications for understanding: (1) the process in which memories are formed, (2) potential therapeutic effects of estrogen in aging especially in menopausal women.